Apparatus and Method for Monitoring and Controlling Extracorporeal Blood Flow Relative to Patient Fluid Status

ABSTRACT

A system and method for controlling extracorporeal blood flow in a patient. The system includes a blood pump having a rotor, a plurality of rollers carried by the rotor and a pump chamber extended in tension about the rollers. A sensor measures an operating parameter of the blood pump and a controller, coupled to the sensor, calculates the flow efficiency of the blood pump based on the measured operating parameter. The controller is further configured to display the flow efficiency on the display device, and the operation of the blood pump is adjusted based on the flow efficiency when necessary.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. provisional applicationentitled “Apparatus and Method for Monitoring and Achieving OptimalExtracorporeal Blood Flow Relative to Patient Fluid Status” havingApplication No. 61/086,467 and filed on Aug. 5, 2008.

BACKGROUND

1. Field of the Invention

The present invention generally relates to mechanical blood pumps. Morespecifically, the invention relates to biocompatible mechanical bloodpumps for circulatory support of patients, particularly those inrefractory cardiogenic shock.

2. Description of Related Technology

Cardiogenic shock occurs in a variety of clinical scenarios, including,but not limited to, acute myocardial infarction, viral myocarditis,massive pulmonary embolus, decompensated cardiomyopathy, and postcardiotomy failure. Without mechanical support, mortality is extremelyhigh and has been reported to be from 40-80 percent. Although manypatients die from multisystem organ failure, low cardiac output islikely the inciting event. The institution of circulatory support andrestoration of end organ blood flow could potentially attenuate organinjury and reduce mortality.

A variety of ventricular assist devices (VADs), including blood pumps,are currently available or in clinical trials. Many of these devices aredesigned for intermediate or long term support, either as a bridge toheart transplantation or as destination therapy. Since these devices areimplanted, they require extensive surgical procedures and are veryexpensive. Because of the expense and complexity of implantation,universal application of these devices to the majority of patients withcardiogenic shock is impractical.

One type of VAD, a roller pump, is frequently used in cardiopulmonarybypass and is always used in association with a venous reservoir, whichensures adequate filling of the pump. In the absence of a venousreservoir, as would be required for prolonged support, significantnegative pressure (with catastrophic cavitation) can result duringinadequate venous drainage. In addition, a sudden occlusion of theoutflow limb of the pump will generate exceedingly high circuitpressures, risking tubing rupture. These features make roller pumpsimpractical for use as VADs, but they are often employed inextracorporeal membrane oxygenation (ECMO) systems, where trainedperfusionists or other personnel typically oversee and operate the pumpcontinuously. Although the disposable costs are low, labor costs formonitoring can make this type of support less cost effective.

As seen from the above, there is a need for a biocompatible, mechanicalblood pump that can be used as a VAD to restore circulation incardiogenic shock, and which is inherently safe, self regulating,nonthrombogenic, simple, durable for weeks, applicable for left, right,or bi-ventricular support, or ECMO, and for patients of any age or size.In addition, the device should be inexpensive and provide for monitoringand achieving optimal extracorporeal blood flow relative to patientfluid status.

SUMMARY OF THE INVENTION

In satisfying the above need, as well as overcoming the enumerateddrawbacks and other limitations of the related art, the presentinvention provides a system for controlling extracorporeal blood flow ina patient. The system comprising: a blood pump including a rotor, aplurality of rollers carried by the rotor and a pump chamber extended intension about the rollers; a sensor configured to measure an operatingparameter of the blood pump; a controller coupled to the sensor, thecontroller calculating flow efficiency of the blood pump based on theoperating parameter measured by the sensor; and a display device coupledto the controller, the controller being configured to display on thedisplay device the flow efficiency as determined by the controller.

According to one aspect of the invention, the flow efficiency iscalculated according to the equation FLOWEFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100, wherein SV_(max)=themaximum stroke volume under maximum filling pressure; Q=output flow ratein liters per minute; RPM=the rotor speed in revolutions per minute; andROLLERS=the number of rollers on the rotor.

According to another aspect of the invention, the controller isconfigured to graphically display the flow efficiency on the displaydevice.

According to a further aspect of the invention, the operating parameteris output flow rate of the blood pump.

According to yet another aspect of the invention, the system includes auser input coupled to the controller and configured to adjust at leastone additional operating parameter of the blood pump, including withoutlimitation RPM of the rotor, vacuum pressure, and pump chamber tension.

According to still another aspect of the invention, the blood pumpincludes three rollers.

According to another aspect of the invention, the pump chamber isoccluded in a free condition where the pressure acting on the interiorof the pump chamber is equal to or less than the pressure acting on theexterior of the pump chamber.

According to a further aspect of the invention, the pump chamber isdefined by a pair of flexible side walls joined at lateral edges thereofand defining a passageway therebetween from a pump inlet to a pumpoutlet.

According to yet another aspect of the invention, the system furthercomprising a pump enclosure defining an interior compartment, theinterior compartment housing the rotor, rollers and pump chambertherein.

According to still another aspect of the invention, the pump enclosureis airtight and the interior compartment is coupled to a vacuum source.

According to a further aspect of the invention, a motor is coupled tothe rotor for rotation thereof, the controller being configured tocontrol rotation of the rotor via the motor, the motor being a DCbrushless motor.

According to still another aspect of the invention, the controller isconfigured to display the rotational speed of the rotor on the displaydevice.

According to another aspect of the invention, a method is provided forcontrolling an extracorporeal blood flow system, the method comprising:providing a extracorporeal blood flow system including a blood pumphaving a rotor, a plurality of rollers carried by the rotor and a pumpchamber extended in tension about the rollers, a sensor configured tomeasure an operating parameter of the blood pump, a display device, auser interface, and a controller coupled to the sensor, the blood pump,the user interface and the display device; measuring an operatingparameter of the blood pump; calculating flow efficiency of the bloodpump based on the measured operating parameter; displaying flowefficiency on the display device as determined by the controller; andvarying the rotational speed of blood pump based on the flow efficiency.

According to yet a further aspect of the invention, the flow efficiencyis calculated according to the equation FLOWEFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100, wherein SV_(max)=themaximum stroke volume under maximum filling pressure; Q=output flow ratein liters per minute; RPM=the rotor speed in revolutions per minute; andROLLERS=the number of rollers on the rotor.

According to another aspect of the invention, the displaying of the flowefficiency on the display device is done in graphical form.

According to yet another aspect of the invention, the measured operatingparameter is output flow rate of the blood pump.

According to still another aspect of the invention, the method furthercomprising the step of applying a vacuum to the exterior of the pumpchamber.

According to a further aspect of the invention, a method of treating apatient using an extracorporeal blood flow system is provided. Themethod comprising: providing a extracorporeal blood flow systemincluding a blood pump having a rotor, a plurality of rollers carried bythe rotor and a pump chamber extended in tension about the rollers, asensor configured to measure an operating parameter of the blood pump, adisplay device, a user interface, and a controller coupled to thesensor, the blood pump, the user interface and the display device;connecting the blood pump of the extracorporeal blood flow system to thevascular system of the patient; measuring an operating parameter of theblood pump; calculating flow efficiency of the blood pump based on themeasured operating parameter; displaying the flow efficiency on thedisplay device as determined by the controller; and varying therotational speed of blood pump based on the flow efficiency.

According to another aspect of the invention, the treating methoddetermines the flow efficiency according to the equation FLOWEFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100, wherein SV_(max)=themaximum stroke volume under maximum filling pressure; Q=output flow ratein liters per minute; RPM=the rotor speed in revolutions per minute; andROLLERS=the number of rollers on the rotor.

According to still another aspect of the invention, the treating methoddisplays the flow efficiency on the display device is in graphical form.

According to yet another aspect of the invention, in the treating methodthe measured operating parameter is output flow rate of the blood pump.

According to a further aspect of the invention, the treating methodfurther comprises the step of applying a vacuum to the exterior of thepump chamber.

Further objects, features and advantages of this invention will becomereadily apparent to persons skilled in the art after a review of thefollowing description, with reference to the drawings and claims, whichare appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a system incorporating theprinciples of the present invention and being directly connected to apatient;

FIG. 2 is perspective view of ventricular assist device, according tothe principles of the present invention, with its enclosure opened toshow the interior compartment and components located therein;

FIG. 3 is a cross sectional view of the pump chamber in its occluded andfree condition, where the pressure inside the pump chamber is equal toor less than the pressure on the exterior of the pump chamber;

FIG. 4 is a cross sectional view of the pump chamber in its pressurizedcondition, where the pressure inside the pump chamber is greater thanthe pressure on the exterior of the pump chamber;

FIG. 5 is a schematic illustration of the of a system incorporating theprinciples of the present invention, apart from being connected to apatient;

FIG. 6 is a table listing various components as may be included in thecontrol unit of an embodiment of the present invention;

FIG. 7 is a graph of the relationship between inlet pressure and flowrate at various rotor speeds;

FIG. 8 is graphical representation of the relationship between inletpressure and flow rate at various rotor speeds for an idealized patientand patients with increased and decreased fluid status relative to theidealized patient; and

FIG. 9 is a diagrammatic illustration of a further embodiment of asystem incorporating the principles of the present invention in abi-ventricular assist application.

DETAILED DESCRIPTION

Referring now to the drawings, a system for providing a ventricularassist device to a patient is generally illustrated in FIG. 1 anddesignated at 10. As seen in FIG. 1, the system 10 is intended to bedirectly connected to the patient without the imposition of a venousreservoir or other such components between the system 10 and thepatient. As illustrated in FIG. 1, the patient is generally depicted bythe schematic representation of a heart, which is designated byreference numeral 12, but which may be other circulatory locations inthe patient. The system 10, while depicted as being placed for leftventricular assist by being coupled to the heart via the placement of anintake catheter 14 in the left ventricle 16 and the placement of anouttake catheter 18 in the aorta 20 of the patient 12, it will beapparent that the system 10 may be used for right ventricular assist,bi-ventricular assist, pulmonary assist, isolated organ perfusion,isolated limb perfusion, hypothermia treatment, cancer treatment, wholebody treatment, partial body treatment, dialysis, apheresis, andperitoneal dialysis (each with its own appropriate location ofattachment of the system to the patient 12).

The system 10 generally comprises of a blood pump, hereinafter aventricular assist device (VAD) 22, a control unit 24, a display 26, anda user interface 28, the latter of which may or may not be integral withthe display 26.

The VAD 22 includes a collapsible conduit, which is herein referred toas the pump chamber 30. The pump chamber 30 is wrapped under tensionaround freely rotating rollers 32 that are themselves mounted on a rotor34 to form a roller assembly 36. The rotor assembly 36 and the pumpchamber 30 may be housed in an interior compartment 38 of an enclosure40.

As seen in FIG. 2, the enclosure 40 preferably includes a movable topwall 42 connected to one or more side walls 44 and a bottom wall 46.When the top wall 42 is closed upon the side walls 44, the top wall 42may optionally use polymeric gaskets or other materials to form anairtight seal of the interior compartment 38, thereby allowing acontrolled vacuum to be applied. Applying such a vacuum to the interiorcompartment 38 allows for vacuum assisted drainage from the patient 12to the system 10, which may be useful in overcoming pressure losses inthe catheter from the patient 12 to the system 10.

For this purpose, a vacuum port 48 is provided in and through one of thewalls 42, 44, 46 of the enclosure 40 and allows the interior compartmentto be coupled to a vacuum pump (V) 69, (e.g., Schwarzer PrecisionSP270EC), which is schematically shown in FIG. 5. If vacuum assisted,the system 10 will preferably produce blood flows up to 5 L/min withinlet siphon pressures of 74 mmHg and maximum outlet pressures of 400mmHg. This performance is suitable for emergency flow requirements ofmost adult patients. An inlet pressure of 74 mmHg is standard for ECMOsupport, which typically relies upon gravity drainage to the regulatedroller pumps. Limiting the outlet pressure to 400 mmHg reduce hemolysisand eliminates the possibility of circuit rupture.

The enclosure 40 is preferably constructed of a biocompatible plasticmaterial such as acrylic, and may be formed by injection molding theside walls 44 and the bottom wall 46 as a single, unitary piece, and thetop wall 42 being a separate component mounted via a hinge 50 or otherconnection to the side walls 44. Alternatively, the various portions ofthe enclosure 40 can be individually formed and joined together by anyappropriate means, such as ultrasonic welding or the use of adhesives.

The pump chamber 30 has a cross sectional shape that is collapsed andoccluded in its “free” condition (i.e., when the pressure inside thepump chamber 30 equals the pressure acting on the exterior of the pumpchamber 30), as seen in FIG. 3. The pump chamber 30 will prime only whenfluid is supplied at a pressure above the pressure exterior to the pumpchamber 30, such as the pressure in the interior compartment 38 of theenclosure 40. When fluid is being provided at such a positive pressure,the pump chamber 30 will begin to assume the lenticular cross sectionalshape generally illustrated in FIG. 4.

To achieve the above occlusion in the free condition, the pump chamber30 is constructed of from two sheets of a naturally flaccid flexiblematerial 52, such as medical grade polyurethane sheets (e.g., StevensMP-1880-P). The two sheets of material 52 are laid one atop the otherand joined at their lateral edges 54 by a radio-frequency (RF) sealingmachine (e.g., J.A. Callanan Co., Chicago, Ill., Model 60SB) weldingthereby forming the pump chamber 30. In one preferred embodiment, thepump chamber 30 is formed with a constant width of 1.9 inches and athickness of 0.023 inches for each sheet 52.

Additionally, as generally represented in FIG. 1, the pump chamber 30may include an inlet tube 56 and an outlet tube 58, of an appropriatelysized PVC tubing (preferably with an inner diameter of the inlet being ½inch and an inner diameter of the outlet being ⅜ inch) RF welded to thelongitudinal ends of the conduit defined by the two joined sheets 52.Fittings can be added to the tubes 56, 58 to locate and secure the tubes56, 58 to enclosure of the VAD 22.

With the pump chamber 30 constructed in this manner, once an inletregion 60 of the pump chamber 30 is primed with fluid, as the rotor 34rotates, fluid will be driven by peristaltic motion toward an outletregion 62 of the pump chamber 30. This pumping action is achievedwithout the use of a stator to occlude the pump chamber at the locationof the rollers. Without a stator, the pressure generated by the VAD 22is limited by the tension of the pump chamber 30 about the rollers 32.At a characteristic pressure limit (e.g., 300-400 mmHg), the fluidwithin the pump chamber 30 simply slips past the rollers and,resultingly, the pressure cannot further increase. Accordingly, onebenefit of the present system is that the VAD 22 will not cause ruptureof the blood circuit if the fluid outlet side of the system 10 isoccluded, kinked or otherwise blocked.

The rotor 34 carries a plurality of freely supported rollers 34, whichmay be mounted on the ends of arms 64. By being freely supported, therollers 32 are free to rotate relative to the rotor 34 and independentof the speed of rotation of the rotor 34. Preferably, the rollers 32 areequally spaced about the rotor assembly 36, are constructed of apolymeric bearing material and supported by polished, stainless steelpins affixed between the ends of the arms 64 of the rotor 34. As seen inthe figures, the rotor assembly 36 includes three rollers 32. Whileillustrated with three rollers 32, it will be appreciated that at leasttwo rollers 32 must be used and that a greater number of rollers 32could be used. The rotor 34 may be formed of machined acrylic or asimilar material formed into the desired configuration.

The rotor 34 is mounted to the output shaft of a motor 66, which ispreferably a brushless DC gear motor (e.g., 1/16 hp, 200 rpm max). Theoutput shaft 68 extends through the bottom wall 46 of the enclosure 40,if an enclosure 40 is provided, and is provided with a sealed bearing ofappropriate construction so as to maintain the sealed integrity of theinterior compartment 38 when the compartment 38 is to be subjected tovacuum. DC torque motors are preferred in that they take advantage ofthe fact that the motor torque increases with the square of the diameterand directly with the motor's length. Thus, a relatively large diametermotor having a small length provides high torque and a compact geometry,which in turn allows for direct drive of the rotor assembly 36 withoutthe use of gear assemblies, thereby avoiding expense, noise, weight andreliability concerns. Such motors 66 deliver smooth operation at lowrotor speeds while in the presence of highly varying torque loads, thelatter of which can be caused by the rollers pressurizing anddischarging fluid from the pump chamber 30. This is an important issueduring periods of unstable vascular volumes, which may be very highlyvariable during the first hours and days of vascular support. Aspreviously noted, the system 10 is designed to be connected directly tothe central vascular of the patient 12 without use of a fluid reservoir,although other components may be included in the system 10 depending onthe particular application to for which the system 10 is being employed.When connected directly to the central vascular, variations in thepreload and afterload of the vascular circuit act directly andimmediately on the VAD 22, potentially affecting the balance of flowbetween the venous return to the VAD 22 and the output of the VAD 22.

Coupled to the VAD 22 is the control unit 24, which is typicallyprovided with a number of individual controllers and drivers toeffectuate operation of the various components of the system 10,including the motor 66, the display 26, the user interface 24, thevacuum pump 69 (if so provided) as well as various sensors, such as aflow sensor 70, a outlet pressure sensor 72 and a vacuum pressure sensorif provided (such as a piezoresistive vacuum pressure transducer (e.g.,Keller Druck Series 2 PR). Such controllers and drivers have applicationspecific functions and offer preconfigured software templates for easyimplementation into the system 10. These components may include thedevices listed in the table presented in FIG. 6. For example, thecontrol 24 may include a motor controller 76 specifically developed forbrushless DC motor control using shaft angle encoder based positionfeedback and sinusoidal commutation to achieve smooth, efficientrotation at low rpm. Alternatively, the components of the control unit24 may have a single controller handling substantially all aspects ofthe system, various components of the control unit 24. The control unit24 may include a printed circuit board 25 so as to provide a unifiedchassis for incorporation of the electronics into the system 10.

Preferably, the settling times for such a motor controller 76 should beless than or equal to 3 seconds for a change of plus or minus 10 rpm.Additionally, the rpm of the motor 66 should be able to be maintained ata target level of rpm with the VAD operating at maximum rpm and theoutlet catheter 18 clamped shut. Finally, the motor 66 should beoperable by the motor controller 76 in the expected range of about 10-75rpm.

As suggested above, other components of the control unit 24 may includea display controller 78, a vacuum controller 80 (if so equipped), and aserial communication bus 82 for internal communications and coordinationof the electronic functions. Additionally, the control unit 24 mayfurther include a power input module 84, provided with anelectromagnetic interference (EMI) filter an additional safety featureso as to meet the safety standards required for medical electricalequipment.

During operation of the system 10, the VAD 22 has an output flowratethat is independent of the after load, at least when operated within thephysiologic pressures of the patient 12. Rather, the VAD 22 is preloaddependent, meaning that the filling pressure determines the amount ofstroke volume that is contained within the pump chamber 30 and deliveredfor each roller pass. As such, the filling pressure is a usefulparameter for automatically balancing the pump flow with the venousreturn to the VAD 22.

If the patient's intravascular fluid volume increases, the rise inpreload pressure will increase the stroke volume of the VAD 22 and withit, the flow output. In the event that the preload pressure issufficient to fully fill the stroke volume, the pump output reaches amaximum value for a given rpm. As long as the venous return remainsbelow this maximum stroke volume for a given rpm, the flows will bebalanced and the VAD 22 will remain responsive to changes inintravascular volume. The graph presented in FIG. 7 illustrates theeffect of various filling pressures on the output of the VAD 22 atvarious rpm speeds. As illustrated in FIG. 7, the filling pressures andit's relation to pump output (Q) is shown for three rotor speeds, namely70 revolutions per minute, 100 revolutions per minute and 130revolutions per minute.

In view of the above, the filling pressure or flow efficiency can beused in a clinical environment to allow the rpm to be set so as toachieve automatic flow balance. Additionally, the actual stroke volumemay be used to monitor this balance, as the intravascular volumechanges. For example, with a given rpm of the VAD 22, a maximum strokevolume can be defined based on bench measurements of the maximum flowunder high filling pressures. Under clinical conditions, actual strokevolume can be derived from measured pump flows and the rpm, andexpressed in terms of flow efficiency by the equation:

FLOW EFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100

wherein SV_(max)=the maximum stroke volume under high filling pressures;Q=output flowrate in liters per minute; RPM=the rotor speed inrevolutions per minute; and ROLLERS=the number of rollers on the rotor.

When less than 100% filling occurs, by monitoring the output flow rateof the VAD 22 via the flow sensor 70, the flow efficiency can becalculated according to the above equation. This flow efficiency canthen be outputted to the display 26 in a graphical presentation, as seenin FIG. 1. Less than 100% filling is an indication that the VAD 22 isoperating below its full stroke volume and indicates that the VAD 22 canand will achieve flow balance, while remaining responsive to changes inthe intravascular fluid status (volume) of the patient 12.

The concept of the preceding paragraph is graphically illustrated inFIG. 8, wherein the flow rate is presented for the preload pressure of a4 inch diameter rotor and a 1.9 inch wide pump chamber at various rpmsettings. IV₁ represents the idealized patient characteristic curve forinitial vascular volume. IV₊ and IV⁻ represent positive and negativechanges to the intravascular volume. The intersection of the rpm curveindicates the operating point for flow rate and preload pressure. Asintravascular volumes change, the intersection point with the rpm curveshifts, thereby providing automatic flow balance to the system. Thus, itcan be seen that the flow efficiency indication is a powerful andinnovative concept that offers a simple, easy to understand means ofcontrolling and monitoring the operation of the VAD 22.

In the present system, the display 24 preferably illustrates the flowefficiency data such that the display accuracy is within plus or minus5% of the calculated values. The user interface 28 may be configured toallow for the display resolution of the percent fill conditions to beadjusted, thereby accommodating finer or coarser increments of flowefficiency changes. Additionally, the user interface 28 and the controlunit 24 will preferably allow for custom system configuration and thesetting of user defined alarm limits 86. Such user defined alarm limitsmay be provided for flow efficiency, as shown in FIG. 1, as well as forthe output flow rate and the outlet pressure of the VAD 22. In additionto the above performance criteria, the display 26 additionally presentsthe current speed of the VAD 22 in rpm. Finally, instead of providingthe user interface 28 on a separate screen, the user interface 28 may beintegrated into the display 26, such as utilizing a colored TFT LCD withtouch screen capabilities.

The VAD 22 utilized in the system 10 of the present invention is aperistaltic pump that has safety features inherent in its design andthat will respond to changes in the hemodynamic status of the patientwith appropriate changes in output flow rate from the system. The aboveare accomplished without the need for manual adjustments to pump speed.Additionally, the system 10 has a low priming volume, prevents backflow,has no thrombogenic stagnation zones, is applicable to pediatric use,and has relatively low costs. Notably, the only disposable component ofthe present system is the blood contacting pump chamber 30.

The VAD 22 utilized in the present invention is a true “Starling” pumpwhere flow rate is dependant upon RPM and filling pressure. The pumpchamber 30, being naturally flat, only pumps whatever blood is deliveredto it by venous drainage of the inlet catheter 14. If blood return islimited or stopped altogether, the pump chamber collapses and flow willdecrease in proportion to the decreased filling. Even if the venous lineis totally occluded (kinked), physically obstructed, or intermittentlysucking into the vascular wall (“chattering”), negative pressure cannotbe generated. As a result, cavitation and cavitation related hemolysiswill not occur. Some of the safety features afforded include: 1) Thepump cannot create suction pressure and requires a positive fillingpressure for pumping to occur. This prevents tissue damage at thecannula site in the event of an interruption in venous flow from thepatient. 2) Flow adjusts naturally to the volume status of the patient,obviating the need for specialized personnel to constantly change thepump speed. 3) The flat free condition nature of the pump chamberminimizes stress in the pump tubing, averting rupture while preventingbackflow in the event of loss of power. 4) The maximum pressuregenerated by the pump is dictated by the tension of the pump chamber 30around the rotor, and is set to ensure that the pump does not generatedangerously high outlet pressures. This feature prevents potentialdisruption of the blood circuit. 5) The peristaltic nature does notrequire use of flow valves, avoiding possible failures and stasis.

In an effort to address size and portability concerns, computationalefforts were made to determine rotor diameter and pump chamber geometry(i.e. width, length, thickness) that would produce flow performancesuitable for temporary adult application. With a pump rotor diameter of4 inches and a pump chamber thickness of 0.023 inches and width of 1.9inches, the VAD 22 can readily produce the flow required for temporaryadult support, namely 5 L/min.

Vacuum assisted drainage will allow positioning of the pump in closeproximity to the patient, reducing priming volume and blood transittime, and facilitating ambulation. A miniature printed circuit board mayinclude pressure transducer amplification with adjustable pressure limitswitches to activate the vacuum pump in the event pressure drops below atarget set point and to stop the vacuum pump when the target level ofvacuum is reached. A vacuum controller may include an adjustablehysteresis to prevent rapid cycling of the vacuum pump around the targetpressure point.

As a further embodiment of the present invention, FIG. 9 illustrates theimplementation of the principles of the present invention in abi-ventricular assist application. In this implementation, a second VAD22 is connected by its intake catheter 14 to the heart 12 via the rightatrium 15 and by its outtake catheter 18 to the pulmonary artery 19. Thetwo VADs 22 utilize with a common controller 24, display 26 and userinterface 28. In all other material respects, the two VADs 22 are of thesame construction discussed above with reference to FIGS. 1-8.

As a person skilled in the art will readily appreciate, the abovedescription is meant as an illustration of implementation of theprinciples this invention. This description is not intended to limit thescope or application of this invention in that the invention issusceptible to modification, variation and change, without departingfrom spirit of this invention, as defined in the following claims.

1. A system for controlling extracorporeal blood flow in a patient, thesystem comprising: a blood pump including a rotor, a plurality ofrollers carried by the rotor and a pump chamber extended in tensionabout the rollers; a sensor configured to measure an operating parameterof the blood pump; a controller coupled to the sensor, the controllercalculating flow efficiency of the blood pump based on the operatingparameter measured by the sensor; and a display device coupled to thecontroller, the controller being configured to display on the displaydevice the flow efficiency as determined by the controller.
 2. Thesystem of claim 1 wherein the flow efficiency is calculated according tothe equationFLOW EFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100 wherein:SV_(max)=the maximum stroke volume under maximum filling pressure;Q=output flow rate in liters per minute; RPM=the rotor speed inrevolutions per minute; and ROLLERS=the number of rollers on the rotor.3. The system of claim 1 wherein the controller is configured tographically display the flow efficiency on the display device.
 4. Thesystem of claim 1 wherein the operating parameter is output flow rate ofthe blood pump.
 5. The system of claim 1 wherein the system includes auser input coupled to the controller and configured to adjust at leastone additional operating parameter of the blood pump.
 6. The system ofclaim 1 wherein the blood pump includes three rollers.
 7. The system ofclaim 1 wherein the pump chamber is occluded in a free condition wherethe pressure acting on the interior of the pump chamber is equal to orless than the pressure acting on the exterior of the pump chamber. 8.The system of claim 1 wherein the pump chamber is defined by a pair offlexible side walls joined at lateral edges thereof and defining apassageway therebetween from a pump inlet to a pump outlet.
 9. Thesystem of claim 1 further comprising a pump enclosure defining aninterior compartment, the interior compartment housing the rotor,rollers and pump chamber therein.
 10. The system of claim 9 wherein thepump enclosure is airtight and the interior compartment is coupled to avacuum source.
 11. The system of claim 1 further comprising a motorcoupled to the rotor for rotation thereof, the controller beingconfigured to control rotation of the rotor via the motor, the motorbeing a DC brushless motor.
 12. The system of claim 1 wherein controlleris configured to display the rotational speed of the rotor on thedisplay device.
 13. A method for controlling an extracorporeal bloodflow system, the method comprising: providing an extracorporeal bloodflow system including a blood pump having a rotor, a plurality ofrollers carried by the rotor and a pump chamber extended in tensionabout the rollers, a sensor configured to measure an operating parameterof the blood pump, a display device, a user interface, and a controllercoupled to the sensor, the blood pump, the user interface and thedisplay device; measuring an operating parameter of the blood pump;calculating flow efficiency of the blood pump based on the measuredoperating parameter; displaying the on the display device the flowefficiency as determined by the controller; and varying the rotationalspeed of blood pump based on the flow efficiency.
 14. The method ofclaim 13 wherein the determining the flow efficiency is calculatedaccording to the equation:FLOW EFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100 wherein,SV_(max)=the maximum stroke volume under maximum filling pressure;Q=output flow rate in liters per minute; RPM=the rotor speed inrevolutions per minute; and ROLLERS=the number of rollers on the rotor.15. The method of claim 13 wherein the displaying of the flow efficiencyon the display device is done in graphical form.
 16. The method of claim13 wherein the measured operating parameter is output flow rate of theblood pump.
 17. The method of claim 13 further comprising the step ofapplying a vacuum to the exterior of the pump chamber.
 18. A method oftreating a patient using an extracorporeal blood flow system, the methodcomprising: providing a extracorporeal blood flow system including ablood pump having a rotor, a plurality of rollers carried by the rotorand a pump chamber extended in tension about the rollers, a sensorconfigured to measure an operating parameter of the blood pump, adisplay device, a user interface, and a controller coupled to thesensor, the blood pump, the user interface and the display device;connecting the blood pump of the extracorporeal blood flow system to thevascular system of the patient; measuring an operating parameter of theblood pump; calculating flow efficiency of the blood pump based on themeasured operating parameter; displaying the flow efficiency on thedisplay device as determined by the controller; and varying therotational speed of blood pump based on the flow efficiency.
 19. Themethod of claim 18 wherein the determining of the flow efficiency iscalculated according to the equation:FLOW EFFICIENCY=(((Q÷RPM)×(1÷ROLLERS))÷SV_(max))×100 wherein,SV_(max)=the maximum stroke volume under maximum filling pressure;Q=output flow rate in liters per minute; RPM=the rotor speed inrevolutions per minute; and ROLLERS=the number of rollers on the rotor.20. The method of claim 18 wherein the displaying of the flow efficiencyon the display device is done in graphical form.
 21. The method of claim18 wherein the measured operating parameter is output flow rate of theblood pump.
 22. The method of claim 18 further comprising the step ofapplying a vacuum to the exterior of the pump chamber.